TWI744063B - Wireless communication system and transmission rate control method - Google Patents

Abstract

A wireless communication system includes a transceiver circuit, a memory circuit, and a processor circuit. The transceiver circuit transmits data via sub-channels (which includes first and second sub-channels). The memory circuit stores a lookup table that indicates corresponding relations between transmission rates and channel indicators. The processor circuit selects a first channel indictor from the lookup table according to a transmission rate of the first sub-channel during a statistics interval, determines at least one difference value between a first channel estimated value of the first sub-channel and a second channel estimated value of the second sub-channel, determines a reference channel indicator according to the difference value and the first channel indicator, and selects a corresponding transmission rate from the lookup table according to the reference channel indicator, in order to set a transmission rate of the transceiver circuit over the second sub-channel to be the corresponding transmission rate.

Description

Wireless communication system and transmission rate control method

This case is about a wireless communication system, especially a wireless communication system using orthogonal frequency division multiple access (orthogonal frequency division multiple access) technology and its transmission rate control method.

In an orthogonal frequency division multiple access (OFDMA) system, a channel can be divided into multiple sub-channels, which are called resource units. The packet transmission rate on these resource units can determine the overall transmission performance of the OFDMA system. In some technologies, the retry data of the transmitting end can be analyzed to set the transmission rate of the resource unit. However, in practical applications, only a small part of the sub-channels will transmit retransmission information, or only a small part of the sub-channels have enough retransmission information for statistics. As a result, these technologies still cannot set an appropriate transmission rate for subchannels that do not transmit retransmission information or for subchannels with insufficient information.

In some embodiments, the wireless communication system includes a transceiver circuit, a memory circuit, and a processor circuit. The transceiver circuit is used to transmit data through a plurality of sub-channels. The sub-channels include at least a first sub-channel and a second sub-channel. The memory circuit is used for storing the first look-up table. The first look-up table indicates the correspondence between a plurality of transmission rates and a plurality of channel indicators. The processor circuit is used for: selecting at least one first channel indicator among the channel indicators from the first look-up table according to at least one first transmission rate of the at least one first subchannel in the statistical period; and determining at least one first subchannel At least one difference between the estimated value of the at least one first channel and the estimated value of the second channel of the second sub-channel; determine the first reference channel index according to the at least one difference and the at least one first channel index; and according to The first reference channel index selects the corresponding transmission rate among the transmission rates from the first look-up table to set the transmission rate of the transceiver circuit on the second subchannel as the corresponding transmission rate.

In some embodiments, the transmission rate control method includes the following operations: establishing a first look-up table, where the first look-up table indicates a corresponding relationship between a plurality of transmission rates and a plurality of channel indicators; according to at least one first of the plurality of sub-channels At least one first channel indicator among the channel indicators is selected from the first look-up table for the at least one first transmission rate of the sub-channel during the statistical period; the at least one first channel estimated value of the at least one first sub-channel and the At least one difference between the second channel estimated values of the second sub-channel in the sub-channels; determining the first reference channel index according to the at least one difference and the at least one first channel index; and according to the first reference channel The indicator selects the corresponding transmission rate among the transmission rates from the first look-up table to set the transmission rate of the transceiver circuit in the second subchannel as the corresponding transmission rate.

With regard to the features, implementation and effects of this case, the preferred embodiments are described in detail as follows in conjunction with the drawings.

All words used in this article have their usual meanings. The above-mentioned terms are defined in commonly used dictionaries, and any examples of the use of terms discussed here in the content of this case are only examples, and should not be limited to the scope and meaning of this case. Similarly, the present case is not limited to the various embodiments shown in this specification.

Regarding the "coupling" or "connection" used in this article, it can refer to two or more components directly making physical or electrical contact with each other, or indirectly making physical or electrical contact with each other, and can also refer to two or more components. Interoperability or action of components. As used herein, the term "circuit" can be a device that is connected in a certain manner by at least one transistor and/or at least one active and passive element to process signals.

As used herein, the term "and/or" encompasses any combination of one or more of the listed associated items. In this article, words such as first, second, and third are used to describe and distinguish each element. Therefore, the first element in this document can also be referred to as the second element without departing from the original intent of this case. For ease of understanding, similar elements in each drawing will be designated with the same reference numerals.

FIG. 1 is a schematic diagram of a wireless communication system 100 drawn according to some embodiments of the present application. In some embodiments, the wireless communication system 100 may use orthogonal frequency division multiple access (OFDMA) technology to connect with other devices (for example, the device 100A).

The wireless communication system 100 includes an antenna 120, a transceiver circuit 140, a memory circuit 160, and a processor circuit 180. The transceiver circuit 140 can transmit data to the device 100A via the antenna 120, or receive data from the device 100A via the antenna 120. In some embodiments, the transceiver circuit 140 is used to transmit data via multiple subchannels. In some embodiments, as shown in FIG. 3, the aforementioned multiple subchannels may be multiple resource units (resource units) RU0 to RU4 under OFDMA technology. In some embodiments, the transceiver circuit 140 may include (but is not limited to) a transmitter circuit (not shown), a receiver circuit (not shown), a baseband circuit (not shown), etc., to transmit and/ Or receive information.

In some embodiments, the memory circuit 160 is used to store a first look-up table LT1, which is used to indicate a correspondence between multiple transmission rates and multiple channel indicators. The processor circuit 180 can use the first look-up table LT1 to set the transmission rate of the sub-channel. The related operations of using the first look-up table LT1 will be described later with reference to FIGS. 2 to 4. In some embodiments, the memory circuit 160 stores one or more program codes, and the processor circuit 180 can execute the one or more program codes to perform multiple operations in FIG. 2 to set the transmission rate. In some embodiments, the wireless communication system 100 may be simulated and/or measured in advance to establish the first look-up table LT1. In some embodiments, the first look-up table LT1 may be the measurement result reported by the device 100A (receiving end). In some embodiments, the first look-up table LT1 may be a test result obtained by the wireless communication system 100 (transmitting end) during channel estimation.

In some embodiments, the first look-up table LT1 can be expressed as the following table (hereinafter referred to as Table 1): Transmission rate Channel indicators MCS0 1 MCS1 4 MCS2 6 MCS3 9 MCS4 12 MCS5 16 MCS6 18 MCS7 20 MCS8 twenty four MCS9 28 MCS10 32 MCS11 36 In the first look-up table LT1 of the foregoing embodiment, the transmission rate in the left column may be represented by a modulation and coding scheme (MCS) index value. The higher the MCS index value, the faster the transmission rate. For example, the transmission rate corresponding to MCS9 is higher than the transmission rate corresponding to MCS8. In some embodiments, the channel indicator in the right column may be (but not limited to) the signal to noise ratio (SNR) corresponding to each transmission rate, and the unit of expression is decibel.

In some embodiments, the first look-up table LT1 can be expressed as the following table (hereinafter referred to as Table 2): Transmission rate Channel indicators MCS0 0 MCS1 3 MCS2 5 MCS3 8 MCS4 11 MCS5 15 MCS6 17 MCS7 19 MCS8 twenty three MCS9 27 MCS10 31 MCS11 35 Compared with the embodiment of Table 1, in the embodiment of Table 2, the channel index may be the difference between the signal-to-noise ratio corresponding to each transmission rate and the signal-to-noise ratio corresponding to the lowest transmission rate. For example, in Table 1, the signal-to-noise ratio corresponding to MCS0 is 1 decibel, and the signal-to-noise ratio corresponding to MCS1 is 4 decibels. Therefore, in Table 2, the channel index corresponding to MCS0 is 0 (that is, 1-1=0), and the channel index corresponding to MCS1 is 3 (that is, 4-1=3). By analogy (for example, the channel indicator corresponding to MCS8 is 24-1=23), you should understand the setting method of the channel indicator in Table 2. In some embodiments, by recording the difference between the channel indicators corresponding to the multiple transmission rates, the channel difference between the multiple transmission rates can be more accurately reflected. Table 1 and Table 2 are only examples, and this case is not limited to this. The first look-up table LT1 for the correspondence between various recordable transmission rates and channel indicators is within the scope of this case.

In some embodiments, the memory circuit 160 may be (but not limited to) a non-transitory computer-readable storage medium. For example, computer-readable storage media include temporary memory, semiconductor or solid-state memory, magnetic tape, removable computer disk, random access memory (RAM), hard disk, optical disk, or the like. In some embodiments, the processor circuit 180 may be (but not limited to) a central processing unit (CPU), an application-specific integrated circuit, a multi-processor, a pipelined processor, or a distributed processing unit. System or its equivalent. Various circuits or units used to implement the memory circuit 160 and the processor circuit 180 are all within the scope of this application.

FIG. 2 is a flowchart of a method 200 for controlling a transmission rate according to some embodiments of the present application. In some embodiments, the transmission rate control method 200 can be executed by (but not limited to) the processor circuit 180 of FIG. 1.

In operation S210, a first lookup table is established, where the first lookup table indicates the correspondence between multiple transmission rates and multiple channel indicators. For example, as mentioned above, the memory circuit 160 may store the first look-up table LT1 as shown in Table 1 or Table 2.

In operation S220, at least one first channel indicator is selected from the first look-up table according to the at least one first transmission rate of the at least one first subchannel in the statistical period.

To illustrate operation S220, please refer to FIG. 3. FIG. 3 is a schematic diagram of drawing the frequency spectrum of the wireless communication system 100 of FIG. 1 and related operations of FIG. 2 according to some embodiments of the present case. As shown in Figure 3, the frequency spectrum is divided into 5 resource units RU0 to RU4. The processor circuit 180 can record the number of packet transmissions of multiple resource units RU0～RU4 during a preset statistical period, and calculate the resource units RU0～RU0～RU0～RU0～RU0～RU0～RU0 through a rate adaptation algorithm or a resource allocation algorithm. The transmission rate of RU4. For example, in the statistical period, the number of packets transmitted via the resource unit RU0 and the data unit RU1 is 0, so the transmission rate of each of the resource unit RU0 and the data unit RU1 is unknown (denoted as X and Y respectively); via the resource unit The number of packet transmissions transmitted by RU2 is 400, and the transmission rate corresponding to the resource unit RU2 is MCS7; the number of packet transmissions transmitted via the resource unit RU3 is 200, and the transmission rate corresponding to the resource unit RU3 is MCS5; transmitted via the resource unit RU4 The number of packet transmission times is 200, and the transmission rate corresponding to the resource unit RU4 is MCS5.

In some embodiments, the processor circuit 180 can select the resource unit with the largest packet transmission frequency among the resource units RU0 to RU4 as the aforementioned at least one first subchannel. As shown in FIG. 3, since the resource unit RU2 has the maximum number of packet transmissions (that is, 400) in the statistical period, the processor circuit 180 can select the resource unit RU2 as at least one first subchannel, and according to the corresponding transmission of the resource unit RU2 The rate MCS7 selects at least one corresponding first channel indicator from the first look-up table LT1 (for example, Table 1 or Table 2 above). For example, the processor circuit 180 may obtain at least one first channel index of 20 from Table 1 according to the transmission rate MCS7 corresponding to the resource unit RU2.

Continuing to refer to FIG. 2, in operation S230, at least one difference between at least one first channel estimated value of the at least one first sub-channel and a second channel estimated value of a second sub-channel of the plurality of sub-channels is determined value. In some embodiments, as shown in FIG. 1, the memory circuit 160 further stores a second look-up table LT2, which is used to indicate channel estimation values of a plurality of sub-channels (ie, resource units RU0 to RU4). The processor circuit 180 can obtain at least one estimated value of the first channel and an estimated value of the second channel according to the second look-up table LT2. In some embodiments, the second look-up table LT2 can be represented as the following table (hereinafter referred to as Table 3), in which the number of packet transmissions and the transmission rate are the same as the statistical data in FIG. 3. Resource unit RU0 RU1 RU2 RU3 RU4 Packet transmission times 0 0 400 200 200 Transmission rate X Y MCS7 MCS5 MCS5 Channel estimate 18 twenty two 26 twenty one twenty two

In some embodiments, based on the beamforming protocol of the OFDMA system, the processor circuit 180 periodically sends a request frame to the device 100A via the transceiver circuit 140, and the device 100A sends back a response to The channel quality indicator measured in this data frame. In some embodiments, the channel quality indicator is used to indicate the signal-to-noise ratio of the corresponding sub-channel (ie, resource unit). The processor circuit 180 may record the corresponding relationship between the channel quality indicators (ie, channel estimated values) and the resource units RU0 to RU4 to establish a second look-up table LT2. In some embodiments, when the transceiver circuit 140 transmits the interrogation frame to the device 100A, the processor circuit 180 may perform channel estimation to obtain the channel estimation value corresponding to each sub-channel and establish the second look-up table LT2. The above-mentioned setting method of the second look-up table LT2 is used as an example, and this case is not limited to this.

In some embodiments, the number of packet transmissions of the second subchannel in the statistical period is less than a preset value (for example, 100) or zero. The above-mentioned default values can be used to select resource units with insufficient statistical information. In the example of FIG. 3, since the packet transmission times of the resource unit RU0 or the resource unit RU1 are all zero and less than 100, the second subchannel can be the resource unit RU0 or the resource unit RU1. The following takes the resource unit RU0 as the second subchannel as an example for description. In the previous operation, the resource unit RU2 is selected as at least one first subchannel. According to the second look-up table LT2 (as shown in Table 3), the processor circuit 180 knows that the estimated value of at least one first channel corresponding to the resource unit RU2 is 26, and knows the second channel estimated value corresponding to the resource unit RU0 The measured value is 18. In some embodiments, the processor circuit 180 may subtract at least one first channel estimated value from the second channel estimated value to determine at least one value between the at least one first channel estimated value and the second channel estimated value. A difference. In other words, in this example, at least one difference is -8 (that is, 18-26=-8).

Continuing to refer to FIG. 2, in operation S240, a first reference channel index is determined according to the at least one difference value and the at least one first channel index. In operation S250, a corresponding transmission rate of one of the transmission rates is selected from the first look-up table according to the first reference channel index, so as to set a transmission rate of the transceiver circuit in the second sub-channel as the corresponding transmission rate.

In some embodiments, since the difference between the estimated channel value and the channel index both correspond to the SNR value, the processor circuit 180 may add at least one difference and at least one first channel index to determine the reference channel index. As shown in the example of FIG. 3, after the processor circuit 180 obtains at least one first channel index of 20 and at least one difference value of −8, the processor circuit 180 may add at least one first channel index and at least one difference value. , To generate the first reference channel indicator. In other words, in this example, the first reference channel index is 12 (that is, 20+(-8)=12). Then, according to the first look-up table LT1 (as shown in Table 1), the processor circuit 180 can select the transmission rate MCS4 corresponding to the first reference channel index (for example, 12) from the first look-up table LT1 to set the transceiver The transmission rate of the circuit 140 in the resource unit RU0 is MCS4.

Through similar operations, the processor circuit 180 can also set the transmission rate of the resource unit RU1. With the transmission control method 200, the processor circuit 180 can use the channel characteristics of at least one first sub-channel (for example, resource unit RU2) with sufficient statistical data to estimate the second sub-channel (for example, resource unit RU2) with insufficient statistical data. The channel characteristics of RU0 and resource unit RU1) are used to set a suitable transmission rate to subchannels with insufficient statistical data.

The above multiple operations are only examples, and are not limited to be performed in the order in this example. Without violating the operation mode and scope of the embodiments of the present case, various operations under the transmission rate control method 200 can be appropriately added, replaced, omitted, or performed in a different order. Alternatively, one or more operations under the transmission rate control method 200 may be performed simultaneously or partially simultaneously.

FIG. 4 is a schematic diagram of drawing the frequency spectrum of the wireless communication system 100 of FIG. 1 and related operations in FIG. 2 according to some embodiments of the present case. Compared with FIG. 3, in this example, the number of packet transmissions of each of the aforementioned at least one first sub-channel in the statistical period is greater than or equal to a predetermined value. In some embodiments, if the number of packet transmissions of a resource unit is greater, the reference value of the statistical data (for example, the transmission rate) related to the resource unit is higher. Therefore, by setting a preset value, multiple resource units with a certain number of packet transmissions can be selected to obtain a more accurate estimation result.

For example, the preset value may be (but not limited to) 100. The processor circuit 180 may select at least one of the plurality of resource units RU0 to RU4 having the number of packet transmission times greater than 100 in the statistical period as at least one first subchannel (ie, operation S220). As shown in FIG. 4, since the packet transmission times of the resource unit RU2, the resource unit RU3, and the resource unit RU4 are all greater than 100, the processor circuit 180 can select the resource unit RU2, the resource unit RU3, and the resource unit RU4 as at least one first child. aisle. The processor circuit 180 can select the corresponding transmission rate MCS7, transmission rate MCS5, and transmission rate MCS5 corresponding to the resource unit RU2, resource unit RU3, and resource unit RU4 from the first look-up table LT1 (for example, Table 1 or Table 2). Channel indicators. For example, the processor circuit 180 obtains at least one first channel index of 20, 16, and 16 from Table 1 according to the transmission rate MCS7, the transmission rate MCS5, and the transmission rate MCS5.

Then, in operation S230, according to the second look-up table LT2 (as shown in Table 3), the processor circuit 180 can obtain a plurality of at least one first channel estimation value corresponding to the resource unit RU2, the resource unit RU3, and the resource unit RU4 They are 26, 21, and 22, and the estimated value of the second channel corresponding to the resource unit RU0 is 18. Therefore, the processor circuit 180 can determine that at least one difference value is -8 (ie 18-26), -3 (ie 18-21), and -4 (ie 18-22).

In some embodiments, in operation S240, the processor circuit 180 may determine a plurality of second reference channel indicators according to at least one difference value and at least one first channel indicator, and perform a weighting operation according to the plurality of second reference channel indicators, To determine the first reference channel index. In some embodiments, the processor circuit 180 may add a corresponding one of the at least one difference value and a corresponding one of the at least one first channel indicator to determine the at least one reference channel indicator. In some embodiments, if one of the at least one first subchannel (hereinafter referred to as the target subchannel) has a greater number of packet transmissions in the statistical period, one of the second reference channel indicators corresponds to (for example, The second reference channel index corresponding to the target sub-channel) has a higher weight in the weight calculation.

其中，0.5、0.25以及0.25分別為對應於資源單元RU2、資源單元RU3以及資源單元RU4的多個權重，12、13以及13分別為對應於資源單元RU2、資源單元RU3以及資源單元RU4的多個第一通道指標。 For example, the processor circuit 180 may add up at least one first channel indicator (ie 20) corresponding to the resource unit RU2 and at least one difference (ie −8) corresponding to the resource unit RU2 to determine the at least one reference channel indicator as 12. By analogy, the processor circuit 180 can determine that the multiple second reference channel indicators corresponding to the resource unit RU2, the resource unit RU3, and the resource unit RU4 are respectively 12 (ie 20 + (-8)), 13 (ie 16 + (-3) )) and 12 (ie 16 + (-4)). Since the packet transmission times of the resource unit RU2, resource unit RU3, and resource unit RU4 are 400, 200, and 200, respectively, the processor circuit 180 can set the weights of multiple second reference channel indicators according to the ratio between the packet transmission times as 2, 1, and 1. Therefore, the processor circuit 180 can use the following formula to obtain the first reference channel index as 12.25:

Among them, 0.5, 0.25, and 0.25 are multiple weights corresponding to resource unit RU2, resource unit RU3, and resource unit RU4, and 12, 13 and 13 are multiple weights corresponding to resource unit RU2, resource unit RU3, and resource unit RU4, respectively. The first channel indicator.

In some embodiments, the aforementioned weighting operation may be a maximum ratio combining operation, but this case is not limited to this. Various appropriate weight calculations are covered by this case.

In operation S250, according to the first look-up table LT1 (for example, Table 1), the processor circuit 180 may select from the first look-up table LT1 the transmission rate corresponding to the channel index that is closest (or the same) as the first reference channel index. In this example, the first reference channel index is 12.25, so the processor circuit 180 can select the transmission rate MCS4 corresponding to the channel index 12 (which is closest to 12.25) to set the transmission rate of the transceiver circuit 140 in the resource unit RU0 For MCS4. Similarly, through similar operations, the processor circuit 180 can also set the transmission rate of the resource unit RU1.

The number of sub-channels, the value of the preset value, the value of the transmission rate, and the value of the channel index mentioned in the above multiple embodiments are all used for example, and the present case is not limited to this. The above multiple values can be adjusted accordingly according to actual application requirements.

To sum up, with the wireless communication system and the transmission rate control method in some embodiments of the present application, an appropriate transmission rate can be set for the sub-channels with insufficient statistical information to increase the overall transmission performance.

Although the embodiments of this case are as described above, these embodiments are not used to limit the case. Those with ordinary knowledge in the technical field can apply changes to the technical features of the case based on the explicit or implicit content of the case, and all such changes All of them may fall into the scope of patent protection sought in this case. In other words, the scope of patent protection in this case shall be subject to the scope of the patent application in this specification.

100: wireless communication system 100A: device 120: Antenna 140: Transceiver circuit 160: memory circuit 180: processor circuit 200: Transmission rate control method S210, S220, S230, S240, S250: Operation LT1: First lookup table LT2: Second lookup table RU0~RU4: Resource Unit

[Figure 1] is a schematic diagram of a wireless communication system drawn according to some embodiments of this case; [Figure 2] is a flow chart of a transmission rate control method drawn according to some embodiments of this case; [Fig. 3] is a schematic diagram of drawing the frequency spectrum of the wireless communication system of Fig. 1 and the related operations of Fig. 2 according to some embodiments of the present case; and [Fig. 4] is a schematic diagram of drawing the frequency spectrum of the wireless communication system of Fig. 1 and the related operations of Fig. 2 according to some embodiments of the present case.

100: wireless communication system

100A: device

120: Antenna

140: Transceiver circuit

160: memory circuit

180: processor circuit

LT1: First lookup table

LT2: Second lookup table

Claims (10)

A wireless communication system, including: A transceiver circuit for transmitting data through a plurality of sub-channels, wherein the sub-channels include at least a first sub-channel and a second sub-channel; A memory circuit for storing a first look-up table, wherein the first look-up table indicates a corresponding relationship between a plurality of transmission rates and a plurality of channel indicators; and A processor circuit for: Selecting at least one first channel indicator from the first look-up table according to the at least one first transmission rate of the at least one first subchannel in a statistical period; Determine at least one difference between the at least one estimated value of the first channel of the at least one first subchannel and the estimated value of the second channel of the second subchannel; Determine a first reference channel indicator based on the at least one difference and the at least one first channel indicator; and According to the first reference channel index, one of the transmission rates corresponding to the transmission rate is selected from the first look-up table to set a transmission rate of the transceiver circuit on the second subchannel as the corresponding transmission rate. For example, in the wireless communication system of claim 1, wherein the at least one first sub-channel is a sub-channel having the largest number of packet transmissions in the statistical period among the sub-channels. For example, the wireless communication system of claim 1, wherein the processor circuit is used to add the at least one difference value and the at least one first channel index to determine the first reference channel index. For example, the wireless communication system of claim 1, wherein the number of packet transmission times of the second subchannel in the statistical period is less than a preset value or zero. For example, the wireless communication system of claim 1, wherein the number of transmissions of a packet in the statistical period of each of the at least one first sub-channel is greater than or equal to a preset value, and the processor circuit is further configured to perform according to the at least A difference value and the at least one first channel indicator determine a plurality of second reference channel indicators, and a weighting operation is performed according to the second reference channel indicators to determine the first reference channel indicator. For example, the wireless communication system of claim 5, wherein if the packet transmission times of a target subchannel in the at least one first subchannel in the statistical period is greater, the second reference channel indicators corresponding to the target subchannel One of the channels has a higher weight in the weight calculation. For example, the wireless communication system of claim 5, wherein the weighting operation is a maximum ratio composite operation. For example, the wireless communication system of claim 1, wherein the processor circuit is used to subtract the at least one first channel estimated value from the second channel estimated value to determine the at least one difference value. For example, the wireless communication system of claim 1, wherein the memory circuit is further used to store a second look-up table, the second look-up table is used to indicate a plurality of channel estimation values of the sub-channels, and the processor circuit is more The method is used to obtain the at least one first channel estimated value and the second channel estimated value according to the second look-up table. A transmission rate control method, including: Establishing a first look-up table, where the first look-up table indicates a corresponding relationship between a plurality of transmission rates and a plurality of channel indicators; Selecting at least one first channel indicator among the plurality of channel indicators from the first look-up table according to at least one first transmission rate of at least one first subchannel in a statistical period of the plurality of subchannels; Determining at least one difference between at least one first channel estimated value of the at least one first sub-channel and a second channel estimated value of one of the second sub-channels of the plurality of sub-channels; Determine a first reference channel index according to the at least one difference value and the at least one first channel index; and The corresponding transmission rate of one of the transmission rates is selected from the first look-up table according to the first reference channel index, so as to set a transmission rate of a transceiver circuit in the second sub-channel as the corresponding transmission rate.

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